1. Field of the Invention
The present invention relates to an image-forming apparatus, particularly one used for a dry type electrophotography. More specifically, it relates to a process for exchanging a developing agent in the above type image-forming apparatus, and a device for carrying out such a process.
2. Description of the Related Arts
In an image-forming apparatus used for dry type electrophotography, an electrostatic image is formed on a photoconductive drum, developed by a developing agent delivered from a developer unit, and transferred onto a paper, and the image on the paper is fixed thereto by a fuser unit.
The developing agent usually consists of two components; a carrier formed of a ferrite powder easily attracted to a magnetic roll, and a toner formed of a synthetic resin, such as a polyester resin or a polyacrylic resin, which can be carried by the carrier to the photoconductive drum and attached thereto. Upon formation of the developed image on the photoconductive drum, and the transfer thereof to the paper, the toner is exhausted but the carrier is returned to a powder container in a developer unit for reuse.
As the toner is expendable, a toner cartridge is inserted in the developer unit, from which the toner is transferred into the container by a feed roller and mixed with the carrier therein by a mixing screw, while being controlled in such a manner that the color density of the toner mixture monitored by a sensor is maintained at a predetermined level.
Although the carrier is not expendable, in principle, it should be periodically replaced because the surface state of the carrier powder initially imparted thereto deteriorates as a result of long term use, which causes a lowered color density or a fogging of a fixed image.
In the prior art, the carrier replacement is carried out by discharging all of the old carrier from the container and filling fresh carrier therein. This replacement includes a complicated operation for the detaching of container from the developer unit, for which expert service personnel are required.